Micro-CT Imaging of Fibers in Composite Laminates under High Strain Bending
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RESEARCH PAPER
Micro-CT Imaging of Fibers in Composite Laminates under High Strain Bending A.T. Zehnder1 · V. Patel1 · T.J. Rose2,3 Received: 24 September 2019 / Accepted: 6 April 2020 © The Society for Experimental Mechanics, Inc 2020
Abstract To be able to track the deformation of fibers in composite laminates subject to high strain bending and to study any changes that occur due to elevated temperature hold, an experimental technique using micron scale X-ray computed tomography (micro-CT) was developed and is reported here. A lab based X-ray micro-CT system with resolution as low as 0.7 μm was used to scan an approximately 0.76 × 0.76 × 0.27 mm volume of a 0.27 mm thick unidirectional carbon fiber/epoxy matrix composite laminate. Long scans of 16 hours at low energy, 40 kV, were needed to yield low noise, clear CT images. The laminate is bent to a radius of curvature of approximately 8.2 mm and held statically in a 3D printed fixture. Since the fixture is in the beam path it is important that it have low and uniform X-ray absorption. Several materials were tried, resulting in the choice of polyamide for the 3D printed fixture. Treating fiber cross sections as particles and adapting an algorithm designed for tracking particle trajectories, individual fibers in the deformed state are identified and their coordinates mapped out. Analysis of the fibers shows no measurable change to the fiber curvatures before and after being held at 80 ◦ C for one hour. Keywords Computed tomography · X-ray · Composite · Carbon fiber · High strain · Bending · Particle tracking
Introduction Thin flexible composites have been shown to sustain surface strains surpassing that of thicker composites due to the relative proximity of the fibers in tension to the fibers in compression [1–4]. With laminate thicknesses of approximately 0.1-0.5 mm there are not enough fibers through the thickness of the composite to fully distribute the stress of either the tension or compression surfaces. This results in the stress states interacting to create a complex stress state, which suppresses the pure compressive buckling failure and allows the laminate to reach a second mode failure. An example of the extreme bending that can be obtained is shown in Fig. 1.
A.T. Zehnder
[email protected] 1
Cornell University, Ithaca, NY, USA
2
Roccor, 2602 Clover Basin, Suite D, Longmont, CO 80503, USA
3
University of Colorado, Boulder, CO 80309, USA
However, for long duration holds it is believed that there is a viscoelastic relaxation of the matrix materials, which allows the fibers to re-orient into lower overall energy states with local curvature extremes. These local extremes are believed to lead to fiber failures [5] and degraded performance of the laminates. The objective of utilizing micro-CT imaging methods is to visually identify the viscoelastic effects on the fiber orientations and relative curvature within thin, flexible composites. Over the last twenty years X-ray computed tomography (CT) [6] has become a widely used tool in materials research
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